Improvements have been made. American military figures show that sectarian killings in Baghdad have decreased substantially. In many of Baghdad’s most battle-scarred areas, including Mansour in the west and Ur in the east, markets and parks that were practically abandoned last year have begun to revive.

The surge has also coincided with and benefited from a dramatic turnaround in many Sunni areas where former insurgents and tribes have defected from supporting violent extremism, delivering reliable tips and helping the Americans find and eliminate car bomb factories. An average of 23 car bombs a month struck Baghdad in June, July and August, down from an average of 42 over the same period a year earlier.

But the overall impact of those developments, so far, has been limited. And in some cases the good news is a consequence of bad news: people in neighborhoods have been “takhalasu” — an Iraqi word for purged, meaning killed or driven away. More than 35,000 Iraqis have left their homes in Baghdad since the American troop buildup began, aid groups reported.

The hulking blast walls that the Americans have set up around many neighborhoods have only intensified the city’s sense of balkanization. Merchants must now hire a different driver for individual areas, lest gunmen kill a stranger from another sect to steal a truckload of T-shirts.

To study the full effects of the troop increase at ground level, reporters for The New York Times repeatedly visited at least 20 neighborhoods in Baghdad and its surrounding belts, interviewing more than 150 residents, in addition to members of sectarian militias, Americans patrolling the city and Iraqi officials.

They found that the additional troops had slowed, but far from stopped, Iraq’s still-burning civil war. Baghdad remains a city where sectarian violence can flare at any moment, and where the central government is becoming less reliable and relevant as Shiite or Sunni vigilantes demand submission to their own brand of law. “These improvements in the face of the general devastation look small and insignificant because the devastation is so much bigger,” said Haidar Minathar, an Iraqi author, actor and director. He added that the security gains “have no great influence.”

The troop increase was meant to create conditions that could lead from improved security in Baghdad to national reconciliation to a strong central government to American military withdrawal. In recent weeks, President Bush and his commanders have shifted their emphasis to new alliances with tribal leaders that have improved security in Diyala Province, the Sunni Triangle and other Sunni areas, most notably Anbar Province.

That area, not Baghdad, was the one Mr. Bush conspicuously chose to visit this week.

But when he announced on Jan. 10 his plan to add 20,000 to 30,000 troops to Iraq, Mr. Bush emphasized that Baghdad was the linchpin for creating a stable Iraq. With less fear of death in the capital, “Iraqis will gain confidence in their leaders and the government will have the breathing space it needs to make progress in other critical areas,” he said.

That has not happened. More than 160,000 American troops are now in Iraq to help secure 25 million people. Across Baghdad — which undoubtedly remains a crucial barometer — American and Iraqi forces have moved closer to the population, out of giant bases and into 29 joint security stations. But even as some neighborhoods have improved, others have worsened as fighters moved to areas with fewer American troops.

Lt. Col. Steven M. Miska, deputy commander of a brigade of the First Infantry Division that is charged with controlling northwest Baghdad, said, “We’ve done everything we can militarily.”

He added, “I think we have essentially stalled the sectarian conflict without addressing the underlying grievances.”

Sunnis and Shiites still fear each other. At the top levels of the government and in the sweltering neighborhoods of Baghdad, hatreds are festering, not healing.

The political standoff identified by this week’s Government Accountability Office report can be found not just in the halls of Parliament. The distrust and obstinacy start in the streets.

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DORA An American soldier watched a woman as she left her house in this tense neighborhood of Sunni extremists. The troop buildup has brought more peace here, but has pushed militants into neighboring Saydia.Credit
Chris Hondros/Getty Images

Dealing with intermittent electricity, few jobs, widespread corruption and fresh memories of unspeakable horrors, Iraqis of all sects are scrambling for power, for control.

Iraq’s mixed neighborhoods are sliding toward extinction. During the troop increase, Shiite militias have continued to drive Sunnis out of at least seven neighborhoods of Baghdad. The Mahdi Army, loyal to the radical cleric Moktada al-Sadr, is turning into what many describe as a shadow government, while desperate Sunnis have come to rely almost exclusively on American troops for their protection — a remarkable turnaround from four years ago when the Americans arrived.

In the minds of many, the fight is for survival. For others, the moment of calm has raised disconcerting questions about Iraq’s societal breakdown and where to go from here. The past seven months have crystallized a sense that the Americans are no longer the primary issue: Sunnis most fear Shiite Iran; Shiites are terrified of Sunni extremists and Baathists.

What Congress must now decide, based on extensive data and testimony from Gen. David H. Petraeus, the top commander in Iraq, and Ryan C. Crocker, the American ambassador, is to what extent an American presence can define Iraq’s future. The fifth and final brigade of the troop buildup arrived only in June. General Petraeus has focused on “tactical momentum,” citing the so-called Sunni awakening as proof of success and cause for a continued and expansive American investment of lives and money.

But a close look at three kinds of neighborhoods — Sunni, Shiite and mixed — indicates that while there is certainly momentum, it is still largely driven by the sectarian forces in Iraq, and moving according to their rules.

In Huriya, a Shiite Takeover

The Sunni mosques in Huriya sit empty, burned and broken, while new monuments to revered Shiite imams have arisen, framed in sparkling black marble.

In that working-class western Baghdad neighborhood, the signs of a Shiite takeover stand out — and offer a glimpse into a possible future of a Shiite-ruled Iraq without a capable, nonsectarian government.

The Sunnis are gone, forced out by the Mahdi Army. And in the wake of that rout — which peaked just before a company of American soldiers moved into a joint security station on Jan. 31 — violence has declined. One or two bodies a week now appear in the streets instead of the 30 or 40 that surfaced weekly in December.

Terror and instability, however, have remained. Here and in nearly every other Shiite-dominated area of Baghdad, from Ur and Sadr City east of the Tigris to Shula west of it, residents and American officials report that the Mahdi Army has expanded and deepened its control of daily life.

Families in Huriya depend on the Sadr organization for gas, medicine and other necessities. In return, many Shiites say they live in constant fear of a knock on the door: sometimes the gunmen come to borrow a car or a house; sometimes they demand help at a checkpoint, or for a mission to kill or displace Sunnis from another neighborhood.

Whatever the militia demands, it gets.

“You have to prove your loyalty to them, otherwise you won’t be safe,” said Lamyia al-Saedi, 31, a Shiite government employee who moved to Huriya eight months ago after being expelled from neighboring Adel, a Sunni stronghold.

American commanders in Huriya (Arabic for “freedom”) recognized the strength of the group’s wide-ranging network soon after their arrival. In early May, a 40-year-old Shiite police officer whose brother had been killed by the Mahdi Army would only agree to talk to American soldiers at 3 a.m., after pulling an officer and a reporter into a dark, unfinished room far from the street.

A week ago, it was much the same: to receive a tip from one of their sources, soldiers from Company A, First Battalion, Second Brigade, 82nd Airborne Division had to wait until everyone else in the neighborhood had gone to sleep.

Some Mahdi leaders have been pushed out, killed or captured, said Capt. George W. Feese, 29, the company commander, but threats from others remain.

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An American soldier tried to keep order while an Iraqi soldier distributed relief packages in Baghdad.Credit
Chris Hondros/Getty Images

Abu Sajat, one of several Mahdi leaders known to Captain Feese’s unit, says he commands several hundred fighters in Huriya, Washash, Iskan and Topchi, a cluster of middle- and working-class areas that have become increasingly violent, and more Shiite, in recent months.

He showed up wearing a brown shirt unbuttoned to his sternum, dark sunglasses and brown polyester pants with a belt that had missed several loops toward the back.

Pulling his belt over a sizable stomach, he bragged that they were playing a game of cat and mouse with the Americans in which the Mahdi Army always has more men, more loyalty among Baghdad’s residents and more freedom of movement. Huriya, he said, was stable because the Sunnis were gone, not because the Americans had arrived.

“They can’t break up our organization,” he said. “If you count all the Americans in Iraq, they are really just prisoners.”

In return for about $120 a month plus “donations” collected from Shiite neighbors or Sunni victims, Abu Sajat said his men had sworn loyalty to Moktada al-Sadr and promised to kill Americans or Sunnis when called upon to do so.

A younger member of the Mahdi Army in Huriya said two men who refused to follow such an order in July ended up dead.

Sunnis remain Abu Sajat’s primary target. After seizing control of roughly 100 Sunni-owned houses in Huriya, Abu Sajat said his men moved on to Iskan and Washash, areas with a lighter American presence to the south. Business has been good — pushing Sunnis out brings in rents from Shiite families moving in, and profits from the sale of furniture or cars.

But Abu Sajat, 36, a former pushcart vendor who said he spent seven years in prison under Saddam Hussein, insisted that he had no interest in money. He said the militia’s earnings from Huriya often went to less fortunate Shiites. Last week, he said his command contributed 23 million Iraqi dinars, or $18,400, to Sadr City families whose homes had been damaged or whose relatives had been killed in American military raids.

His justification for attacking Sunnis was simple, and sectarian: “Their houses belong to us,” he said. “They’ve colonized us for more than 1,000 years.”

“Sunnis are just like the puppies of a filthy dog,” he said. “Even the purest among them is dirty.”

The Americans soldiers in Huriya acknowledge that trying to dismantle the Mahdi network has been a struggle. Several months ago, a photograph of another Mahdi leader, Haider Kadhim, — “the No. 1 action guy in Huriya,” a soldier said — hung on the walls of the windowless joint security station where they live. He was someone whom the soldiers hoped to arrest or kill. Last week, his mugshot was still there.

Abu Sajat said Mr. Kadhim was busy in Topchi, out of the unit’s reach.

Captain Feese, the Company A commander, said Huriya residents felt safer without thugs like Mr. Kadhim on the streets. But even with the extra troops, there are parts of Baghdad, like the northern neighborhood of Shula, where militias roam with impunity.

There, at one of its refugee camps, the Mahdi Army now brazenly issues laminated badges to those it deems worthy of admittance.

A recent American report concluded that Mahdi Army leaders in Shula enjoy “freedom of movement” in part “because of a lack of permanent CF presence,” referring to coalition forces.

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While aspiring police in Sadr al-Yusufiya were screened, an American soldier kept watch.Credit
Marko Georgiev for The New York Times

Colonel Miska, who oversees Shula, Huriya and other Shiite-dominant areas, said units regularly entered the neighborhood for raids, which had killed or captured many prominent Mahdi fighters. But, he said, referring to joint security stations, “We do not have a J.S.S. in Shula, due to lack of combat power.”

In Huriya, Captain Feese’s men have tried to erase the militia’s signs of strength. They have not touched the new Sadr monuments, but they initially tore down posters of Mr. Sadr at the market, only to see them reappear.

Many Iraqis, Captain Feese said, hesitate to work closely with the Americans because “they know I’m going home.” Even now, most Iraqis in Huriya still do not believe that the Americans can protect them in a city where, two weeks ago, the Shiite head of a neighborhood just southeast of Huriya was shot dead in a Mahdi-controlled Shiite area. The killing was understood to be punishment for working with the Americans.

A roadside ditch here in the Sunni triangle town of Sadr al-Yusufiya contains the two extremes of the American experience in that Euphrates River farming village southwest of Baghdad.

At one end sit the remains of a truck bomb that a suicide bomber from Al Qaeda in Mesopotamia tried to ram into the village’s American base in June. At the other end, a thousand overheated Sunni men, ages 18 to 35, wait to be given physical examinations and literacy tests by the very same American troops some of them were trying to kill recently.

The push-ups, pull-ups and reading exams are the American military’s attempt to screen the candidates and hasten their hoped-for entry into the Shiite-dominated Iraqi police.

Those Sunnis now hope the Americans will help them rejoin the new Iraqi order that they rejected, and that has in turn rejected them for so long.

But it remains unclear whether an Iraqi government dominated by religious Shiites will be eager to embrace the large-scale return of these young men of fighting age. Nor is it clear whether the Americans’ new allies of convenience will submit to the Shiite authorities in Baghdad.

Many of the men’s fathers and tribal leaders were officers in the Baath government’s military.

Whatever the suspicions harbored by Prime Minister Nuri Kamal al-Maliki’s Shiite-led government — and the suspicion is entirely mutual — it is the Sunni belt that has produced the most tangible fruit for Mr. Bush and General Petraeus.

Young men in fluorescent-banded jackets stand on every corner, operating checkpoints as part of a growing neighborhood watch venture that General Petraeus has seized upon and branded “Guardians” or “Concerned Citizens.”

Under the project, financed by the American military, the local tribes are paid $10 a day per man to provide security in their areas.

Despite protestations from United States commanders that they are not arming those “volunteers,” local American officers confirm that the sheiks can spend the contract money as they wish, diverting money from wages to buy weapons, radios or vehicles if they choose.

The “awakening,” as it has been called, has brought early dividends. Suicide bomb attacks in Baghdad are down — partly because those areas manufactured bombs and sent them into the capital. Certainly, life at Patrol Base Warrior Keep in Sadr al-Yusufiya has become much easier for Capt. Palmer Phillips and his men of Company B, Second Battalion of the Second Brigade Combat Team, 10th Mountain Division, out of Fort Drum, N.Y.

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HURIYA An Iraqi crouched after being stopped by an American soldier looking for Mahdi Army fighters. The Sunni mosques in this working-class neighborhood are burned and broken, replaced by Shiite monuments.Credit
Johan Spanner for The New York Times

There have been only two roadside bomb explosions in the last five months, and this week they drove their Humvees without incident to and from sheiks’ houses late at night along country roads that only a few months ago would have been treacherous.

“We are now getting information from the local volunteers,” said Captain Phillips. “They are telling us very specific things about Al Qaeda’s activities. They are very specific about checkpoints, people, ratlines and targets.” (Ratlines are supply lines.)

But there are undercurrents.

On a visit to Sadr al-Yusufiya last month, Lt. Gen. Raymond T. Odierno, the second-ranking American commander in Iraq, met with the local sheiks.

They made it abundantly clear that their cooperation did not come free, and that they wanted tangible benefits like jobs, weapons, vehicles, military supplies and electricity.

They also delivered not-so-veiled warnings that the Americans’ low-paying job-creation plan, while welcome, was unlikely to keep their people on the right side of the law for long.

“We are determined to kick out terror from our areas but, sir, you shouldn’t forget that fighting terrorism has to be balanced,” cautioned Yassin Abed al-Gurtani, a former general in Saddam Hussein’s army.

“After a month or a few months, what if this contract ends and people find themselves without jobs and can’t join the police or the army?” he said. “We are worried that they might join the wrong side, and we return back to square one.”

Some have already reverted. Fueling concern over the Americans’ eagerness to embrace untested new allies, Captain Phillips concedes that he has already arrested 15 of the Concerned Citizens “for suspected Al Qaeda ties.” Most were turned in by their own checkpoint colleagues for “facilitating” the movement of wanted men. Captain Phillips says he is convinced that most of the Sunnis genuinely want to get back into the system, in which 85 percent of the National Police force is Shiite, and is encouraged that the Iraqi government sent a senior reconciliation official to talk to them.

But there has been little action. And he points out that there is simply no government near Sadr al-Yusufiya for the Sunnis to turn to, even now that they want to. American officers in areas where similar arrangements have been in place longer say that the Sunni groups lack training and are already growing frustrated with the slow process of being accepted by the Shiite-dominated police.

As yet undeterred, the Sunnis in Sadr al-Yusufiya spend all day in line, baking in the 100-degree temperatures, desperate for jobs.

Others show little interest in national policing, saying they simply want to defend their local areas and are sick of being unable to go even to nearby Mahmudiya without risk of being killed by Shiite militias.

Just as many Shiites instinctively mistrust the minority that kept them down during the Baathist era, so the Sunnis in Sadr al-Yusufiya show little sign of remorse, or of desiring reconciliation.

Many bridle at criticism of Saddam Hussein. Heads shake at the mention of Hussein-era mass graves, including one at Mahawil, just half an hour’s drive south. “Saddam just put bad people in jail,” said Abu Ali, 40. “Some people, they exaggerate.”

Almost all predict an intensified civil war once the Americans leave. “We will fight the government until the very last bullet,” threatened one, before dashing inside to try to join it.

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SADR AL-YUSUFIYA Hundreds of aspiring policemen showed up for screening at an American base. Many Sunnis here hope that the Americans will help them rejoin the new Iraqi order that they had long rejected.Credit
Marko Georgiev for The New York Times

Sahar Naeem Suleiman, 27, went further. “If we get into the Iraqi police we can move to Mahmudiya and Yusufiya and south Baghdad to free them and kill all the militias.”

Col. Michael Kershaw, commander of the Second Brigade Combat Team, 10th Mountain Division, denied that the United States was simply arming one side for future conflicts in Iraq, saying that wider factors were at work.

“Are they playing us?” he said. “To some extent we are all playing each other, right? Everybody’s self-interest is at the center.”

“Somewhere they made a political decision that Al Qaeda does not meet their self-interest,” he added. “All Iraqis see that the United States’ time here is numbered. There is a finite amount of time the United States is going to stay here and do this, and they are going to have to figure this out eventually on their own.”

Militants Quit Dora for Saydia

Visible from any high point in Baghdad are twin pillars of smoke that rise from an oil refinery and a power station near the city’s southern edge. The eastern plume rises from Dora, an area of industry, spacious homes and brutal Sunni extremists, while the western plume edges closer to an area of mixed neighborhoods, including Saydia.

The wide Hilla highway has long acted as a border, keeping the bloodshed of Dora from the stability of Saydia — until the troop buildup. Nineteen-ton Stryker vehicles have hammered through Dora over the past few months, bringing enough peace for a third of the shops in Dora’s main market to reopen.

But the push drove Sunni militants out of Dora and into Saydia, where they began attacking Shiites. It was not long before Shiite militias, including the Mahdi Army, responded in kind, despite repeated calls by Mr. Sadr for a freeze in violent activity. Saydia descended into chaos.

With that, its status as a tolerant, middle-class district shared by Sunnis and Shiites was lost, following the precedent of so many other Baghdad neighborhoods.

Saydia is now a virtual ghost town, where the Shiite Mahdi Army and Sunni Arab death squads roam the streets. If Iraq’s civil war has stalled in other parts of the city, in Saydia the engines of sectarianism are running at full throttle.

The hostilities now reach down to the next generation. “I have been attacked personally by children as young as 10 and 13 with hand grenades and so forth,” said Lt. Col. Barry Huggins, a Stryker commander, describing the fight for Dora earlier this summer.

The once-ruling Sunnis have become even more hostile toward what they now regard as a triple occupation of their country: by Americans, Iranians and Shiites who have seized power from them.

“The most dangerous thing is the government and the people representing the government,” snarled Abu Hashemi, 48, a Dora resident.

He said, “If you ask anyone in Dora, ‘Do you prefer an American soldier or someone belonging to the Mahdi Army or the Badr Brigades?’ they will say ‘Leave the American and kill the other.’ Because the Americans will leave Iraq but the other people are staying with us.”

American commanders are worried about their staying power, too. Some of Col. Ricky Gibbs’s Stryker battalions are rotating out, he told senior Iraqi and American officers last month, and he wants more Iraqi police officers sooner than training units say they can be properly screened, drilled and equipped.

Even if the additional troops stay on, Dora’s gains seem fragile. The restoration of order is still too brittle to persuade the people who matter the most: the thousands of Dora residents who have fled the violence, Sunni and Shiite alike.

Sahira, 40, is a Shiite, but her two daughters are Sunnis, like her former husband. Now living in Karada, she was too afraid to give her full name. She hopes to return to Dora one day, but her children have no such expectation.

“All the residents have left our area; it’s completely empty,” said one of her daughters, Noor, 20. Nodding in agreement the other daughter, Sura, 23, said she was afraid to admit at checkpoints that she is Sunni. “Shias are trouble,” she said, heedless of her mother sitting opposite her. “They kidnap Sunnis at checkpoints.”

Wincing slightly, Sahira rallies but has caught the gloomy mood.

“They can’t control Dora, despite the existence of the American bases,” she said. For now, the fight has simply moved to Saydia — another example of the whack-a-mole problem that the American military has struggled to overcome in Iraq since 2003.

Qassem Hussein Jasem, a 40-year-old Sunni who fled Saydia two months ago, laid the blame for his neighborhood’s collapse squarely on the surge.

“It was a good area until Operation Imposing the Law began to chase the terrorists and outlaws and they started infiltrating from Dora and Bayaa,” he complained.

It was here that Mr. Jasem watched as Sunni extremists sprayed graffiti on the walls threatening, “Leave the Neighborhood.” In response Shiite slogans sprang up, including, “Long Live the Wolf Brigade,” a National Police unit widely feared by Sunnis.

No one is more aware of the police’s sectarian reputation than the American military transition teams that advise them. They are frank about infiltration by the Mahdi Army, known as Jaish al-Mahdi or JAM.

“Because the National Police are influenced or afraid to stop JAM, they will basically turnstile them and let them drop down, conduct any Sunni missions, and then come back,” said Maj. Andy Yerkes, an American adviser of the Iraqi police. “We know it happens.”

There are successes. Some Iraqi commanders instill pride and discipline in their units, the Americans say. In July, one Saydia policeman, Capt. Mushtaq Hassan, rescued a 9-month-old girl from a house where a death squad had just killed her family.

Captain Mushtaq dismissed Sunni accusations of killings and torture as the smears of enemies “trying to ruin our reputation because the Iraqi National Police has eliminated two-thirds of the terrorists.”

Pressed, the most he would concede was that some “individuals” had done “bad things,” but he insisted that they were exceptions.

Few are convinced. First Sgt. Timothy Johnson’s experience of the National Police is particularly stark. Driving in mid-June past a National Police checkpoint, Sergeant Johnson, a 43-year-old from El Paso, waved at the smiling Iraqis he knew well, and received friendly waves back.

Barely 50 feet later a sophisticated roadside bomb known as an explosively formed penetrator hit the rear of his Humvee, missing the crew but blowing his luggage out into the road. The same smiling police officers promptly stole his computer, mobile phone and camera and demanded a $40 bribe to give the computer back.

“I don’t trust them,” he said. “They will smile in your face and stab you in the back. They were just too close to that E.F.P. not to have known.”

Asked if things have improved since then, he shook his head emphatically.

“No, they are the same,” he said. “It’s bad and it’s not going to get better. We’re not going to make a difference, not in the short term. Maybe if we stayed here forever.”